Nashorn will not be fully backward compatible with Rhino. Nashorn will be very ECMA compliant, but will not implement some Mozilla and/or Rhino-specific extensions. So not all Rhino-based projects will be able to easily migrate. But it's not hard to imagine that Nashorn will divert attention and resources away from Rhino.

(I believe I got the above information from the MP4 of the JavaOne presentation that can be found here: https://oracleus.activeevents.com/connect/sessionDetail.ww?S...)